The present invention refers to a coaxial antenna, and in particular to a coaxial antenna with a vertical half-wave dipole having a lower section formed as sleeve and an upper section formed as radiating element, with the sleeve and the radiating element cooperating to resonate at a first frequency or first frequency range, wherein the radiating element is constructed as to form a second half-wave dipole resonating at a second higher frequency or frequency range.
U.S. Pat. No. 2,184,729 and the publication "Antennenbuch", Karl Rothammel, 8th edition, Stuttgart, Telekosmos-Verlag, chapter 25.1.1. "Die Koaxialantenne", disclose a coaxial antenna which is utilized as vertically polarized omnidirectional antenna in the very high frequency range for mobile stations. In a 60-.OMEGA.-coaxial cable with thick inner conductor, the outer sheath, outer conductor and dielectric are withdrawn over a length of electrically .lambda./4 (about .lambda./4.times.0.97) for exposing the bare inner conductor. Placed over the coaxial cable is a copper pipe or brass pipe which is also a quarter wavelength long and soldered to the outer conductor of the cable on the end facing the inner cable conductor. The pipe may be of any diameter so long as the clear width allows a placement of the pipe over the outer sheath of the cable. A coaxial antenna of this type constitutes basically a vertical half-wave dipole, with the lower .lambda./4 section simultaneously forming a quarter-wave sleeve for creating the symmetry.
This conventional coaxial antenna is suitable for transmission and reception of only a single frequency or frequency range, with a ratio equaling an integral multiple.
Further conventional multiple frequency antennas are known which are formed by two vertically superposing half-wave dipoles, with the feeding point being arranged between these half-wave dipoles. Such a multiple frequency antenna must be mounted via a mast which extends parallel to the superimposed half-wave dipoles and includes a jib which extends transversely to the mast for attachment of the multiple frequency antenna. Since such an assembly has a great demand for space, it is not suitable for mobile stations. Moreover, a substantial impairment is encountered because the coaxial cable, which is connected to the antenna, extends parallel to the antenna axis along the mast.